Recently, there has been an increasing demand for miniaturization and polyfunctionalization of electronic equipments, and in view of this trend, various attempts have been made to miniaturize electronic equipments by incorporation thereinto of multi-layer printed circuit boards.
The multi-layer printed circuit boards referred to above have such a construction, for example, as shown in FIG. 6, wherein epoxy resin cured layers 12 and printed circuits 13 are alternately laminated between two sheets of cladding panels 11, 11.
The cladding panel 11 mentioned above usually comprises a copper foil 11a and a resin layer 11b formed on the copper foil 11a. The cladding panel 11 is prepared by laminating a prepreg for forming a resin layer on the copper foil 11a and then curing the prepreg; the prepreg is prepared as a sheet by incorporating reinforcement materials such as glass fiber to a base resin, such as phenolic resin, epoxy resin or polyester resin.
The multi-layer printed circuit boards may be prepared, for example, by the following procedure: the above-mentioned cladding panel 11 having the resin layer 11b, an epoxy resin prepreg 20, a printed circuit 13 and another epoxy resin prepreg 20 are laminated to one another in the manner as shown in FIG. 7, the resulting laminate is sandwiched between 2 pieces of metal plates 21, cushioning materials 22 are individually placed between the press hot plate 23 and jig 21, the epoxy resin prepregs 20 are cured by pressing the laminate by means of two pieces of the press hot plates 23 while heating, and the laminate thus integrated is perforated, followed by through-hole plating and surface etching.
When the multi-layer printed circuit board is prepared in the manner as mentioned above, it is desirable that the surface of the resin layer 11b of the cladding panel 11 is roughened in order to improve adhesion between said resin layer 11b and the epoxy resin cured layer 12 obtained by curing the epoxy resin prepreg 20.
From this reason, cited above, the surface of the resin layer 11b has heretofore been roughened by pressing a film, which has a rough surface and is capable of maintaining its surface roughness even under application of heat against the surface of a prepreg for forming the resin layer 11b of the cladding panel 11 while heating.
Known heretofore as a film used for roughening the surface of the resin layer 11b in the manner as mentioned above is a biaxially oriented film of 1-vinylfluoride polymer of Du Pont, U.S.A. known under a trade name of Tedlar (containing 5% CaCO.sub.3 ; hereinafter called Tedlar film), or a matt film obtained by roughening the surface of a triacetyl cellulose film by means of sand blast.
However, when a cladding panel having a resin layer having a roughened surface is prepared by pressing these conventional matt films to a prepreg for forming a resin layer of the cladding panel while heating, there is observed sometimes such a problem that the filler such as CaCO.sub.3 or the dispersing agent for the filler contained in the films migrates to the resin layer or copper foil contacting with said matt films, thereby decreasing the performance of a multi-layer printed circuit board prepared by using this cladding panel.
Under the circumstances, the present inventors proposed in JP-A-32031/1987 a poly-4-methyl-1-pentene film having a roughened surface as the matt film, by the use of which solves the above mentioned problem associated with the conventional matt films.
However, when the surface of the resin layer of the cladding panel 11 is roughened by pressing this matt film of poly-4-methyl-1-pentene to a prepreg for forming the resin layer of the cladding panel while heating, there are sometimes observed such troubles that the matt film expands and wrinkles, or softens in heat and embossed portions on the surface of the matt film are destroyed by the pressure, whereby the surface of the resin layer 11b of the cladding panel 11 cannot be roughened sufficiently.
Accordingly, there has been earnestly desired the advent of such matt films free from wrinkling or free from destruction of embossed portions thereof during the course of the above-mentioned pressing and heating, and which have excellent releasing properties.